This invention relates to a drive assembly for a top driven drilling machine. More particularly, the invention relates to a positive lock to prevent unwanted tightening or loosening of the drive assembly when using the drill motor to break the connection between the drive assembly and a drill string.
Conventional rotary drilling requires the use of a rotary table, a motor mounted on or below the rig floor for rotating the table, and a four or six sided kelly for rotationally connecting the table to the drill string. In recent years, these drilling machines are being replaced by or retrofitted with top driven drilling machines, otherwise known as power subs or power swivels, which rotate the drill string using a drilling motor suspended from a traveling block within a standard derrick or mast. The drilling motor is connected to the drill string for powered rotation by a cylindrical stem or drive assembly extending downwardly within the derrick from the drill motor. A cutting tool or bit is placed at the bottom end of the drill string which, through the rotational energy supplied by the drill motor, cuts through the earth's formations and deepens a well.
As the well is drilled, the bit becomes worn and periodically must be replaced. When replacement of the bit becomes necessary, a portion of the drill string corresponding in length to one or more sections of drill pipe must be removed from the well and pulled above the drill rig floor. This portion of the drill string, i.e., a stand of pipe, is removed and stored on the rig. The drill string is again pulled from the well exposing the next stand of pipe above the floor and this stand is similarly removed. This sequence, usually referred to as tripping out, is continued until the entire drill string is removed from the well. The bit on the bottom pipe section is replaced and the drill string is reassembled; i.e., tripping in, by connecting all the pipe sections previously removed.
When a drill string must be removed from a well, the driller operates a console on the rig floor causing a traveling block to elevate the drill string. The drill string is lifted until the bottom tool joint of the stand to be removed is just above the rig floor. Slips or wedges are then placed around the drill string and under the tool joint in the slip bowl for supporting the weight of the drill string. The stand is removed from the drill string by first disconnecting the upper end of the stand from the lower end of the drive assembly. With a conventional power swivel, a powered wrench, i.e., a make-break, is used to disconnect the stand. The make-break includes lower grabs for securing the upper end of the stand and an upper wrench for engaging the lower end of the drive assembly. The make-break rotates the upper wrench relative to the lower grabs to loosen the threaded connection therebetween. The upper wrench and lower grabs then must be disengaged and the drive assembly is rotated under the power of the drill motor to disengage the threaded connection, with whatever frictional drag remaining in the threads being reacted downwardly through the stand toward the rig floor. The bottom tool joint is then broken in a similar manner and unthreaded. The stand of pipe is then removed from the drill string and placed in a vertical storage rack.
Notable disadvantages when using an upper wrench to untorque and break the connection between the drill pipe and drive assembly are the added initial cost of equipment, additional maintenance during drilling, and more time required to break connections. It has been proposed to use the drill motor on top driven drilling machines to make and break connections between the drive assembly and the drill pipe. Unlike the kelly on conventional drilling units, the drive assembly for the top driven drilling machines may include one or more joints formed by an internal blow out preventer valve, a telescopic sub, a saver sub, and the like. Since tool joints and drive assembly joints are normally threaded in the same direction, any attempt to break the connection between the drive assembly and drill pipe using the drill motor can simultaneously cause untorquing or unthreading of the joints within the drive assembly, which should remain tight. To insure that only the proper joint breaks loose when using the top drive drill motor, drillers may tighten the remaining joints to 40% or more of the recommended makeup torque. This excessive loading may exceed the capacity of overtorqued joints. Joints may also become tightened during drilling if torsional shocks are present.